Some vacuum systems use water to help maintain the suction seal. If yours is one of them, then the water flow is fairly low and should be passing through a backflow device. That device can create that sound when a minimal amount of water is flowing through it.

: In the dental office I work there is a high pitched noise that no plumber or service person can resolve. The noise will go away if you turn on the water or turn off the suction pump for our evacuation system. Any ideas?